1. Field of the Invention
The present invention relates to an OLED (organic light emitting diode) panel obtained by forming an OLED on a substrate and sealing the OLED between the substrate and a cover member. The invention also relates to an OLED module in which an IC including a controller, or the like, is mounted to the OLED panel. In this specification, xe2x80x98light emitting devicexe2x80x99 is the generic term for the OLED panel and for the OLED module. Electronic equipment using the light emitting device is also included in the present invention.
2. Description of the Related Art
Being self-luminous, OLEDs eliminate the need for a backlight that is necessary in liquid crystal display devices (LCDs), and thus they are most suitable when manufacturing thinner devices. Also, the self-luminous OLEDs are high in visibility and have no limit in terms of viewing angle. These are the reasons for the attention that light emitting devices using the OLEDs are receiving in recent years as display devices to replace CRTs and LCDs.
An OLED has a layer containing an organic compound (organic light emitting material) that provides luminescence (electroluminescence) when an electric field is applied (the layer is hereinafter referred to as organic light emitting layer), in addition to an anode layer and a cathode layer. Luminescence obtained from organic compounds is classified into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). A light emitting device according to the present invention can use one or both types of the light emission.
In this specification, all the layers that are provided between an anode and a cathode together make an organic light emitting layer. Specifically, the organic light emitting layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer. etc. A basic structure of an OLED is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order.
The problem in putting a light emitting device into practice is lowering in luminance of OLED which accompanies degradation of its organic light emitting material.
Organic light emitting materials are weak against moisture, oxygen, light, and heat, which accelerate degradation of the organic light emitting materials. Specifically, the rate of degradation of an organic light emitting material depends on the structure of a device for driving the light emitting device, characteristics of the organic light emitting material, materials of electrodes, manufacture process conditions, how the light emitting device is driven, etc.
Even when the voltage applied to the organic light emitting layer is constant, the luminance of the OLED is lowered as the organic light emitting layer degrades, and an image displayed therefore becomes unclear. In this specification, a voltage applied to an organic light emitting layer from a pair of electrodes is called an OLED drive voltage (Vel).
When an image is displayed in color by using three types of OLEDs that respectively emit red (R) light, green (G) light, and blue (B) light, different organic materials are used to form organic light emitting layers of OLEDs of different colors. Accordingly, the rate of degradation of organic light emitting layer may vary between OLEDs of different colors. Then the difference in luminance between OLEDs of different colors will be noticeably large as time passes, making it impossible for the light emitting device to display an image in desired colors.
The temperature of organic light emitting layer is influenced by the outside temperature and heat generated from the OLED panel itself. Generally, the amount of current flowing in an OLED varies depending on the temperature. FIG. 26 shows a change in voltage-current characteristic of an OLED when the temperature of its organic light emitting layer is changed. With the voltage kept constant, the OLED drive current is increased as the temperature of the organic light emitting layer rises. Since the OLED drive current is in proportion to the OLED luminance, the luminance of the OLED becomes higher as the OLED drive current becomes larger. Since a change in temperature of the organic light emitting layer thus causes a change in OLED luminance, displaying an image in desired gray scales is difficult and current consumption of the light emitting device is increased accompanying a temperature rise.
Generally, temperature change brings varying degrees of changes in OLED drive current to different types of organic light emitting materials and, therefore, in color display, the luminance could be changed by temperature change differently for OLEDs of different colors. It is impossible to obtain desired colors when OLEDs of different colors lose their luminance balance.
The present invention has been made in view of the above, and an object of the present invention is therefore to provide a light emitting device capable of keeping the luminance constant and displaying an image in desired colors without being influenced by degradation of its organic light emitting layer or by temperature change.
The present inventors have taken notice of the fact that the luminance of OLED is lowered by degradation less when light is emitted with a current flow to an OLED kept constant than when light is emitted with the OLED drive voltage kept constant. (In this specification, a current flowing into an OLED is called an OLED drive current (Iel).) Then, the present inventors have thought of preventing a change in OLED luminance due to degradation of OLED by controlling the OLED luminance with current instead of voltage.
Specifically, a current mirror circuit composed of transistors is provided in each pixel in the present invention. The current mirror circuit is used to control the OLED drive current. A first transistor and a second transistor of the current mirror circuit are connected such that the drain current of the first transistor is kept substantially equal to the drain current of the second transistor irrespective of the load resistance value.
A drain current I1 of the first transistor is controlled by a signal line driving circuit. The amount of drain current I1 of the first transistor is always equal to the amount of drain current I2 of the second transistor irrespective of the load resistance value. Accordingly, the drain current I2 of the second transistor is controlled by the signal line driving circuit.
The second transistor is connected to an OLED with a single or plural circuit elements interposed therebetween, so that the drain current I2 thereof flows into the OLED. Therefore the value of OLED drive current flowing into the OLED is controlled by the signal line driving circuit irrespective of the load resistance value. In other words, the OLED drive current can be controlled to have a desired value without being influenced by difference in characteristics of transistors or degradation of OLED.
With the above structure, the present invention can prevent the luminance of OLED from lowering even when the organic light emitting layer is degraded and therefore can display a clear image. If the light emitting device is to display an image in color using OLEDs of different colors and the rate of degradation of organic light emitting layer varies between the OLEDs of different colors, the present invention is capable of keeping the luminance of light of different colors balanced and display in desired colors.
Furthermore, the present invention can set the OLED drive current to a desired value despite a change in temperature of the organic light emitting layer due to the outside temperature and heat generated from the OLED panel itself. Since the OLED drive current is in proportion to the OLED luminance, the luminance of OLED can be prevented from changing and current consumption accompanying a temperature rise can be avoided. If the light emitting device is to display an image in color, the luminance of the OLEDs of different colors can be prevented from changing to keep the luminance of light of different colors balanced and display in desired colors.
Generally, temperature change brings varying degrees of changes in OLED drive current to different types of organic light emitting materials and, therefore, in color display, the luminance could be changed by temperature change differently for OLEDs of different colors. However, the light emitting device of the present invention can obtain a desired luminance irrespective of temperature change to thereby keep the luminance of light of different colors balanced. An image thus can be displayed in desired colors.
In a common light emitting device, the electric potential of a wiring line used to supply a current to pixels is slightly lowered as the wiring line becomes longer because of the resistance of the wiring line itself. This electric potential is lowered to widely varying degrees depending on an image to be displayed. When the ratio of higher gray scale pixels to all of the pixels that receive a current from the same wiring line is large, in particular, the current flowing through the wiring lines is increased in amount to make lowering of electric potential noticeable. When the electric potential is lowered, a smaller voltage is applied to the OLED of each pixel to reduce the amount of current supplied to each pixel. Therefore, the amount of current supplied to one pixel is changed as well as the gray scale number thereof when the gray scale number of other pixels that receive a current from the same wiring line as the one pixel is changed, making it impossible for the one pixel to keep a constant gray scale. In the light emitting device of the present invention, on the other hand, a measured value and a reference value are obtained to correct the OLED current each time a new image is displayed. Therefore, a desired gray scale number is obtained for every new image through correction.
In the light emitting device of the present invention, a transistor used in a pixel may be one formed from single crystal silicon or may be a thin film transistor formed from polycrystalline silicon or amorphous silicon.